When to Disable SIP ALG to Fix VoIP Issues

SIP ALG is a technology that is available on most commercial routers, which is a networking device that forwards traffic to the Internet, or between local area networks (LANs).

The “SIP” part refers to Session Initiation Protocol. This is an underlying service that powers all VoIP activity. For example, SIP registers devices, ensures call presence, and governs call audio.

The “ALG” part refers to Application Layer Gateway. This is a network address translation (NAT) tool that changes private IP addresses and ports into public IP addresses and ports. It functions as a proxy to re-establish the destination addresses in data packets, which improves connectivity. (Note: sometimes ALG is referred to as an application layer gateway instead of application-level gateway, but it’s the same thing).

Why Use SIP ALG?

In theory, SIP ALG is fine. It functions to prevent firewall-related issues on the router by inspecting data packets, and changing them as necessary so that they transmit properly. Based on this, SIP ALG seems like a great idea. After all, who doesn’t want better VoIP connectivity and quality? So, why would you even think about disabling your SIP ALG? Read on to learn more.

Improved Network Security

SIP ALG (Application Layer Gateway) is a network security feature that helps to improve the security of Voice over IP (VoIP) networks. It works by inspecting and modifying SIP messages at the application layer of the network stack. This allows it to detect and block malicious traffic, protect against hijacking, and prevent spoofing, and other threats. It also provides better control over VoIP traffic flow.

Improved Quality of Service (QoS) for VoIP Traffic

SIP ALG helps to improve the quality of service (QoS) for VoIP calls by allowing routers to more effectively manage the bandwidth and resources used for VoIP calls. It can identify packets from different types of calls, prioritize them over other network traffic, and apply QoS rules that are specific to VoIP traffic. This can improve the audio quality of VoIP calls and reduce latency. It can also help to ensure that VoIP traffic is given priority when congestion occurs.

Simplified Configuration and Management

As mentioned previously, SIP ALG assists in the automatic configuration of network address translation (NAT) settings, port forwarding, and firewall rules, eliminating the need for manual intervention and reducing the chances of configuration errors. By transparently inspecting and modifying SIP packets, SIP ALG ensures that SIP-based VoIP calls can traverse network boundaries seamlessly. This simplifies the deployment and management process for VoIP administrators, allowing them to focus on other critical aspects of the system while SIP ALG takes care of the necessary network adjustments in a streamlined manner.

Why Disable SIP ALG?

In practice, SIP ALG can potentially be problematic. This is because it can modify packets in unexpected ways, which corrupts them and makes them unreadable. Despite its potential benefits, the reality is that SIP ALG often causes more problems than it solves. In particular, it can cause issues with IP phone registration, call quality, and audio distortion. These problems are typically caused by the router’s inability to properly interpret SIP messages, resulting in miscommunication between the phone and the service provider. Before you disable your SIP ALG, read on about what this can lead to:

Call drops

One of the most noticeable signs of SIP ALG interference is frequent call drops during VoIP conversations. These drops can occur randomly or at specific intervals, disrupting the call and causing frustration for the users.

Audio Quality Issues

SIP ALG can introduce various audio problems in VoIP calls. These issues may include choppy or distorted audio, sudden volume changes, delays, or even complete loss of audio during conversations. The voice may sound robotic or fragmented, making it difficult to have a smooth conversation.

One-Way Audio

Another common problem associated with SIP ALG is the occurrence of one-way audio. This means that only one party can hear the other while the other person cannot hear anything. It can be confusing and hinder effective communication.

Call Termination Issues

SIP ALG may interfere with the proper termination of calls. When a call ends, the SIP session should be closed cleanly, but with SIP ALG enabled, the session may not be terminated correctly. This can result in lingering connections that affect subsequent calls and may cause issues with call routing.

Failed Call Setup

SIP ALG can interfere with the establishment of VoIP calls. It may modify or discard SIP signaling messages, leading to failed call setup attempts. Users may experience continuous ringing, a lack of response, or unsuccessful call initiation despite valid credentials and network connectivity.

Inability to Receive Incoming Calls

SIP ALG can sometimes block incoming calls or prevent them from reaching the intended recipient. This issue can occur if SIP ALG modifies the SIP headers or fails to forward the incoming call correctly. As a result, callers may receive a busy tone or go straight to voicemail.

NAT Traversal Problems

Network Address Translation (NAT) is a technique used to map private IP addresses to public IP addresses. SIP ALG can interfere with NAT traversal, making it difficult for VoIP traffic to pass through routers and firewalls. This can lead to connectivity issues and hinder communication.

Should You Disable SIP ALG?

Now that we have briefly described SIP ALG, let’s answer the next big question: should you disable SIP ALG for your VoIP system? As with many questions, the only right answer at this time is: “It depends.”

Because there can be many variables, depending on your network, disabling SIP ALG is not always the definitive solution.

If you are experiencing any of the problems listed above, then disabling SIP ALG could be a simple, easy fix. However, if you aren’t experiencing any of these problems, then it should be fine to leave it on (and on most commercial routers, SIP ALG is enabled by default).

How to Disable SIP ALG

If you do decide to disable SIP ALG on your commercial router, then once again, we have some good news for you — and maybe some bad news. Let’s start with the positive.

The process for disabling SIP ALG will vary depending on your specific router, so it is important to consult the manual or contact your service provider for instructions. It should be easy to access your router’s interface. Just look for a decal on the router, which should display the IP address (often or and default login information (often the default login is “admin” and the default password is “password”).

Of course, once you log in you will want to change both the username and especially the password to something unique and secure. Remember, default passwords are very common and can easily be guessed or even found online with a simple search. Updating security settings and using strong passwords is always a good idea to keep your home or business network and devices safe.

Now, for the (potentially) bad news: some router manufacturers make it much harder than it should be to disable SIP ALG. The website GetVOIP.com has created a handy table that provides steps for some popular routers such as TP-Link and Netgear. If you need more support, then consult your router manual, or reach out to the manufacturer.

Advice for Carolina Digital Phone Customers

Depending on your situation and your network router settings you may not have to do anything at all. Start by contacting your IT department and letting us know the problems that you’re experiencing (e.g. diminished call quality, dropped calls, etc.), and our experts will quickly troubleshoot the issue and determine the root cause. If the culprit turns out to be SIP ALG, then we will disable it on your commercial router (we can do this remotely and do not need to visit on-site).

If you are not a Carolina Digital Phone customer, consult with an IT professional about your particular issues. Before you start making changes on your network, make note of what changes you make, should you need to go back and restore things.

Additional Questions ABout Disableing SIP ALG?

We hope that you have found this look at SIP ALG interesting and informative. If you have any additional questions or want a demo of our all-in-one VoIP phone system, then get in touch with us. We’re always here to help! Give us a call at (336) 346-6000

Learn how to connect your team anywhere!

Tell us a little about your business and we will have one of our professionals contact you quickly to answer your questions.